Although prolonged activity of some biologically active (bioactive) polypeptides can be achieved by parenterally administering only very small doses, others are required in sufficient serum concentrations and/or have such a short half-life in serum that a substantial dose (e.g. at least about 100 mg) must be administered to provide the desired biological effect over an extended time such as a week or longer. Somatotropins (growth hormones) are an example of such polypeptides.
To prevent undesirably rapid release into an animal's bloodstream, certain polypeptides have been parenterally administered in liquid vehicles which may optionally contain hydration retardants (antihydration agents) or in association with metals or metal compounds that further lower their solubility in body fluids. To avoid the need for unacceptably large quantities of such a vehicle, substantial concentrations of the polypeptide in the vehicle would be advantageous. However, most bioactive polypeptides are very viscous in substantial concentrations and consequently difficult to inject or otherwise administer in such concentrations. Moreover, many commonly used antihydration agents add viscosity and can diminish convenient injectability of such compositions. For those and other reasons, the right combination of (1) a fast enough release to provide the desired biological effect in the animal, (2) a slow enough release to sufficiently prolong the effect, (3) a dose adequate for release at the required rate over the prolonged period of time and (4) a volume small enough and viscosity low enough for convenient administration has normally been very difficult to achieve.
Since each polypeptide is different, e.g. in its three-dimensional structure and its interaction with other substances, the feasibility of achieving a prolonged effective release with a high loading of polypeptide in a suitable vehicle is impossible to predict or demonstrate theoretically. Yet in many cases, such prolonged release compositions must be developed if the biological activity of the polypeptide is to be provided in a useful, economical fashion.